1. Field of the Invention
The present invention relates to a gate valve apparatus provided between a vacuum chamber and an evacuation pump in a semiconductor manufacturing system, etc.
2. Description of the Related Art
A patent document 1 [Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 9-178000] discloses a slide valve (gate valve) including a housing formed of an upper housing part and a lower housing part that are connected and secured to each other along a separation plane. The lower housing part is formed with a straight exhaust flow path (exhaust passage) for providing communication between an inlet port and an outlet port. A slide plate (valve element) is provided so as to be movable (rectilinearly or pivotally) in a direction perpendicular to the center axis of the exhaust flow path. The movement of the slide plate is effected by a servomotor. The flow rate through the exhaust flow path is controlled by the movement of the slide plate. In the lower housing part, an annular seal ring is provided parallel to the slide plate so as to surround the exhaust flow path. An annular projection is formed on the inner peripheral portion of the seal ring. A first O-ring is disposed in an annular groove provided in a surface of the seal ring located adjacent to the slide plate. A second O-ring is provided on the outer peripheral surface of the annular projection and abuts the inner wall surface of the exhaust flow path.
The gate valve as provided between a vacuum chamber and an evacuation pump operates as follows. When the vacuum chamber is subjected to maintenance (e.g. repair, replacement, cleaning, etc. of component parts), the main valve (comprising the slide plate and the valve seat) is fully closed to bring the pressure in the vacuum chamber to the atmospheric pressure while the vacuum pump is being driven continuously. To carry out film deposition, etching or cleaning, the opening of the main valve is adjusted to control the flow rate for the purpose of controlling the pressure in the vacuum chamber, and the vacuum pump is driven. The first O-ring and the second O-ring are exposed to exhaust gas (containing plasma and radicals) in the exhaust flow path no matter which position the main valve is in, i.e. fully closed position, fully open position, or intermediate position.
In vacuum process chambers of semiconductor manufacturing systems and the like, many methods of decomposing compounds by plasma are used for film deposition, etching, cleaning, etc. Decomposition of a compound by plasma produces radicals (free radicals and atoms having unpaired electrons). The radicals have heat energy and exhibit very high reactivity. Even if they collide against the wall surface of the vacuum chamber several times, the radicals are unlikely to lose their energy. Consequently, the radicals in the exhaust gas decompose and deteriorate polymerized sealing materials such as packings.
Regarding the plasma resistance of seals, if the seals are located at a position where they cannot directly be viewed from the plasma source, deterioration due to plasma of high energy can be prevented to a certain extent because ionized atoms lose the electric charge by collision in the chamber. Accordingly, the influence of the plasma reduces. However, deterioration cannot completely be prevented. The seals are subjected to radicals, which do not readily degrade, in addition to the plasma. Therefore, the deterioration of the seals is a serious problem.
Among seals, a sealing material used in a static sealed portion of a vacuum chamber, etc. is fitted in a seal groove formed in a wall surface of a metal or the like. In order to reach the sealing material, plasma and radicals need to move several millimeters along the metal or other wall surface through a gap of several tens of microns. During the movement, the plasma and radicals lose their energy. Accordingly, the deterioration of the sealing material is reduced.
However, a sealing material of a gate valve used in an exhaust system is located in close proximity to an exhaust gas passage and hence exposed in the midst of radicals having intense reactivity and plasma having a certain level of energy. Therefore, the lifetime of the sealing material is reduced considerably.
One generation ago, fluororubber (FKM) was used as a sealing material for gate valves of apparatuses using plasma. However, at present it has become common practice to use fully fluorinated rubber (perfluoroelastomer: FFKM), which has improved resistance to plasma and radicals, as a sealing material for gate valves in order to cope with the present situation that the energy of plasma and radicals has increased to a high level. Even if fully fluorinated rubber is used, the sealing material is deteriorated by plasma and radicals. It is necessary to replace the deteriorated sealing material once two or three months, depending on the circumstances. The cost of sealing materials is as follows. Assuming that the cost of nitrile-butadiene rubber (NBR) is 1, the cost of fluororubber is from 10 to 20, and the cost of fully fluorinated rubber is from 100 to 200. Thus, it costs a great deal to replace the ultra-costly sealing material of fully fluorinated rubber, and a great deal of loss is caused by suspension of the apparatus due to replacement.